Field of the Invention
This invention relates to a resonant acoustic sensor, and in particular, to an acoustic sensor having a substantially disc shaped acoustic cavity with substantially circular end walls.
Description of Related Art
Devices which determine the composition of a gas mixture by measuring the speed of sound in that mixture are well known in the prior art. The two most prevalent acoustic techniques are the time-of-flight technique and the resonant cavity technique. The drawbacks and limitations of each type of device have been described in the prior art (EP 0813060) and will be summarised again here.
A time-of-flight device (for example: U.S. Pat. No. 5,060,506 and U.S. Pat. No. 5,627,323) uses a pair of transducers to transmit and receive short (typically on the order of microseconds) pulses of acoustic energy. The speed of sound is determined by measuring the time taken for these pulses to travel a known distance through the test fluid. In the devices referenced above, the composition of a binary gas mixture is determined from this measurement. Typical problems with such devices include signal attenuation, echoes, dimensional stability, poor temperature compensation, parasitic conduction, and poor pulse shaping and pulse shape distortion. These problems limit both the performance and long-term stability of time-of-flight devices.
Resonant cavity devices (for example: U.S. Pat. No. 3,848,457, EP 0813060, and U.S. Pat. No. 6,378,372) measure the resonant frequency of an acoustic cavity. When the acoustic cavity is filled with a fluid, the resonant frequency of the cavity directly relates to the composition of the fluid. A key limitation of the devices described in the prior art is the difficulty of efficiently exciting a single dominant mode of resonance in the cavity. Many competing resonant modes may exist within the sensor including radial, axial, longitudinal and azimuthal modes. This complicates the interpretation of the output of the sensor. Poor coupling between the, typically longitudinal, motion of the transmitting transducer (transmitter) into the resonant mode of the cavity, and poor coupling between the resonant mode of the cavity and the receiving transducer (receiver) produces a weak signal. Sources of electrical noise in the receiver such as parasitic signals passing from the transmitter to the receiver through the structure of the device, or mechanical resonance of components such as the diaphragm can be of a magnitude comparable to the signal itself.
The shortcomings of the above resonant sensors are shared by devices based upon a Helmholtz oscillator, for example the prior art disclosed in Appl. Phys. Lett., Vol. 82, No 25, Page 4590. In such a device the air in the neck of the aperture of a cavity vibrates causing pressure oscillations in that cavity which oppose the motion of the air in the neck, leading to simple harmonic motion. In the embodiment described by the above publication, the change in acoustic intensity resultant from changing density and speed of sound of the fluid in the cavity is used to measure the composition of a mixture of hydrogen and air. This design has the additional disadvantage that the single aperture in the resonant cavity prevents a flow of fluid from passing through the sensor cavity, which is desirable for fast response times and ease of integration into a fluidic system.
Given the shortcomings of time-of-flight and existing resonant cavity speed-of-sound sensors, there is a need for speed-of-sound sensor capable of efficiently generating a resonant oscillation significantly larger than competing resonant modes and parasitic oscillations. The efficient generation of a large amplitude radial mode pressure oscillation at the resonant frequency of the cavity overcomes many of the limitations of the prior art.
The efficient generation of resonant acoustic standing waves has been addressed in the field of fluid pumping. Patent applications WO2006/111775, WO2009/112866, WO2010/139916, and WO2010/139918 disclose pumps having substantially disc shaped cavities with high aspect ratios (i.e. the ratio of the radius of the cavity to the height of the cavity) which in operation generate a resonant acoustic standing wave in those cavities.
The pump disclosed in FIG. 1 of WO2006/111775 has a substantially cylindrical cavity 11 comprising a side wall 14 closed at each end by end walls 12, 13. The pump also comprises an actuator 20 that drives one or both of the end walls to oscillate in a direction substantially perpendicular to the surface of the plane of the end walls, referred to hereinafter as “axial oscillations”. In this geometry, the mechanical stiffness of the actuator is well matched to the acoustic impedance of the cavity, enabling efficient generation of a high amplitude pressure oscillation.
The efficient generation of a pressure oscillation in such a cavity is further dependent on the matching of the spatial profile of the fluid oscillation in the cavity and the motion of the driven end wall. When the spatial profiles are well matched, work done by the actuator on the fluid in the cavity adds constructively, thereby enhancing the amplitude of the pressure oscillation in the cavity and delivering improved pump efficiency, referred to herein as mode-shape matching. Conversely, in a pump where the spatial profiles are poorly matched, work done by some regions of the end wall on the fluid reduces rather than enhances the amplitude of the fluid pressure oscillation in the fluid within the cavity. Thus, the useful work done by the actuator on the fluid is reduced and the pump becomes less efficient.
The above concepts are applied here to the design of a resonant acoustic sensor where both the transmitter (which is driven) and the receiver (which is passive) are operatively associated with opposing end walls of a disc shaped cavity. As a result of this geometry, in operation both the mechanical stiffness of the transmitter and the receiver of such a device are well matched to the acoustic impedance of the disc shaped volume of fluid in the cavity. This disc shaped geometry is also suitable for achieving good spatial matching between the displacement profiles of the transmitter and receiver and the radial fluid pressure oscillation in the cavity. The combination of these properties enables efficient generation of a high amplitude pressure oscillations by the transmitter and efficient generation of an output signal from the receiver, overcoming many of the limitations of the prior art.